In transceivers for frequency division duplex (FDD) communication (e.g. a transceiver of a cellular radio equipment), the receiver typically experiences strong interference signals from the transmitter of the same transceiver.
The interference signal from the transmitter has a carrier frequency at duplex distance from the carrier frequency of the receive signal. A typical duplex distance is small compared to the carrier frequencies. Typically, the duplex distance may be less than 100 MHz while the carrier frequencies may, for example, be somewhere between 700 MHz and 3 GHz.
To be able to operate with required performance (e.g. achieving good sensitivity), the receiver should preferably be shielded (or isolated) from the interference from the transmitter of the transceiver, both from transmitter signals at transmit frequency and transmitter generated interference at receive frequency. It is also desirable that the transmitter is shielded (or isolated) from the received signals. Example reasons include that as much of the received energy as possible should be transferred to the receiver for optimal receiver performance and that received signals occurring at the transmitter output may cause interference to the signal to be transmitted.
Such isolation is typically achieved by off-chip acoustic wave duplex filters (duplexers) which may typically consist of two band pass filters, e.g. implemented as Surface-Acoustic-Wave (SAW) filters which have a fixed operation frequency
A drawback with duplexers is that they are typically expensive. They are also bulky which increases the size of a transceiver implementation. Duplexers are also fixed in frequency, which necessitates several duplexers to be used if several frequency bands are to be supported. These problems are becoming more pronounced as the number of frequency bands to be supported by a communication device is increased.
Therefore, there is a need for multi-band (preferably at least partly integrated) solutions that provide isolation between a transmitter and a receiver.
A typical on-chip isolation implementation is based on cancellation of the interferer signal. To achieve perfect cancellation of transmit signals at the receiver input, symmetry is necessary. For this purpose, such solutions typically comprises balancing with a dummy load that equals the antenna impedance both at the receive frequency and at the transmit frequency. Typically, balancing may provide good isolation between transmitter and receiver. Further, a balancing approach may be able to provide a desired function over a large number of frequency bands.
However, if the antenna impedance is complex (inductive or capacitive) and/or varies over time (e.g. due to frequency changes and/or changing antenna surroundings), implementation of a perfect cancellation becomes cumbersome, e.g. since the dummy load must track the antenna impedance at both receive frequency and transmit frequency simultaneously. Furthermore, in solutions using a dummy load, approximately 3 dB of the power will be lost.
“An on-chip wideband and low-loss duplexer for 3G/4G CMOS radios”, by M. Mikehemar, et al., 2010 IEEE Symposium on VLSI Circuits (VLSIC), pp. 129-130 discloses a narrowband autotransformer duplexer with a balancing network (comprising a variable resistor in parallel with a variable capacitor) to cancel transmitter current at receiver input.
“A Tunable Integrated Duplexer with 50 dB Isolation in 40 nm CMOS”, by M. Mikehemar, et al., 2009 IEEE International Solid-State Circuits Conference (ISSCC), pp. 386-387 discloses the center tap of an autotransformer connected to an antenna, and the RX and TX connected to the two sides of the autotransformer. A balancing resistor is connected between the TX and RX.
US 2010/0035563 A1 discloses an RF transceiver front-end that includes a balancing circuit and a multiple node isolation and coupling circuit. The multiple node isolation and coupling circuit includes an auto-transformer and two optional capacitors (each coupled in parallel with a respective winding of the auto-transformer). The antenna is coupled to the common node of the auto-transformer windings, the receiver section is coupled to a first node of one of the windings and the transmitter section is coupled to a node of the other winding. The balancing circuit is coupled to the receiver section and the transmitter section and includes a resistor and a variable capacitor in parallel with each other.
US 2011/0064004 A1 discloses a radio frequency (RF) front-end comprising a power amplifier (PA), a noise-matched low-noise amplifier (LNA), a balance network, and a four-port isolation module. The isolation module isolates the third port from the fourth port to prevent strong outbound signals received at the third port from saturating the LNA coupled to the fourth port. Isolation is achieved via electrical balance.
Similarly as described above, a drawback common to all of these solutions is that the balancing network needs to track impedance changes in the antenna during operation to enable sufficient isolation. The impedance needs to be tracked at both receive frequency and transmit frequency simultaneously. Thus, the implementation is sensitive and complex.
A drawback of these solutions is that they suffer from a high insertion loss, i.e. approximately 3 dB of the power of receive and transmit signals will be lost due to the matched impedance (in particular the balancing resistor) of the balance network. Consequences of the increased loss include that the receiver noise figure (NF) is degraded and that the transmitter power consumption is increased.
The bandwidth of the balancing network solutions may also be limited, which may cause problems when the transceiver front-end is intended to handle a wide range of frequencies.
Therefore, there is a need for alternative and improved (preferably at least partly integrated) multi-band solutions that provide isolation between a transmitter and a receiver.